Snoqualmie Pass

Special Avalanche Bulletin
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Avalanche Warning

Extreme or high avalanche danger occurring or expected to occur within 24 hours: at least high danger
in all three elevation bands.

Avalanche Watch

Warning conditions expected within 12-48 hours. Forecasters expect the probability of a Watch to
increase to Warning conditions in the majority of cases within the above timeframe.

Special Avalanche Bulletin

Unusual conditions meriting special attention that does not meet Watch or Warning criteria, generally
issued outside of the daily forecast season.

Issued: 2:58 PM PST Monday, May 22, 2017

by Dennis D'Amico

NWAC avalanche forecasts apply to backcountry avalanche terrain in the Olympics, Washington Cascades and Mt Hood area. These forecasts do not apply to developed ski areas, avalanche terrain affecting highways and higher terrain on the volcanic peaks above the Cascade crest level.

NWAC Spring Forecast Schedule

The NWAC issued daily mountain weather and avalanche forecasts through Saturday, April 15th. Mountain weather and avalanche forecasts were issued during the spring transition April 20-22nd and April 27-29th. Weekend outlooks will be issued Thursdays, May 4th, 11th, 18th and 25th.

Special advisories, watches and warnings will be issued throughout the spring for unusual or dangerous avalanche conditions. You can find out what constitutes a special advisory, watch or warning here.

Watch for the lack of an overnight refreeze of surface snow, wet snow deeper than boot top and initial pinwheels and initial small loose wet snow avalanches that indicate an increasing loose wet avalanche danger. It is always a good plan to be away from avalanche terrain by the warmest midday and afternoon hours. Careful snowpack evaluation and cautious routefinding will be essential through Tuesday.

Aspect/Elevation

The Aspect/Elevation diagram describes the spatial pattern of the Avalanche Problem by aspect (the
direction a slope faces) and elevation band (Above, Near, or Below Treeline). The diagram will be
filled with black where the Avalanche Problem may exist. You can view the diagram as you would a
mountain on a topographic map. The outer ring represents the Below Treeline elevation band, middle
ring Near Treeline, and the inner ring Above Treeline. The diagram is oriented like a compass, with
the top wedges representing north aspects, the left wedges representing west, etc.

Likelihood

Likelihood is a description of the chance of encountering a particular Avalanche Problem. It combines
the spatial distribution of the Problem and the sensitivity or ease of triggering an avalanche. The
spatial distribution indicates how likely you are to encounter the Problem in the highlighted
avalanche terrain. The sensitivity indicates how easy it is to trigger avalanches including both
natural and human triggered avalanches.

Size

Size is based on the destructive potential of avalanches.

SMALL avalanches are relatively harmless to people unless they push you into a terrain trap.

LARGE avalanches could bury, injure or kill a person.

VERY LARGE avalanches could bury cars, destroy a house, or break trees.

HISTORIC avalanches are even more destructive, and nearing the maximum size the slope could
produce.

Recent images from NWAC:

Avalanche Summary:

Recent Weather

The first half of last week was unusually cool with significant accumulating snowfall in the Olympics and Cascades, with the highest snowfall totals on or near the volcanoes. Strong upper level ridging over the last few days has allowed a steady increase in freezing levels over the weekend peaking to around 12000-13000 feet on Monday. Overnight temperatures have been well above freezing at NWAC weather stations the last few nights.

Recent Avalanche Observations

Loose wet slides on steeper slopes have been the story over the last few days as expected. Larger slides were observed in areas that received more recent snowfall last week.

Forecast for Tuesday:

Weather Outlook

Strong upper level ridging Monday and Tuesday will continue the recent warmth with freezing levels holding steady in the 11,000-14,000 ft range along with mostly sunny days and mostly clear nights. However, a deep low pressure system in the NE Pacific will approach the B.C. coast Monday night and shift the ridge inland on Tuesday, allowing for some cooling in the Olympics and north Cascades during the day on Tuesday. Crest level westerly winds are forecast to increase during the day Tuesday for all areas. As the upper low passes inland and to our north Tuesday night, much cooler air will push into the region along with marine clouds along the west slopes of the Cascades and Olympics and bring about a few light showers.

Avalanche forecast and travel advice

Many steeper aspects have already produced loose wet avalanches over the last few days. On steeper slopes that have not released yet, the loose wet avalanche potential remains highest during the warmer, sunny daytime hours. Large or very large loose wet snow avalanches are likely on the volcanoes where there has been the most recent snow. Remember that even small, loose wet snow avalanches can be powerful and dangerous especially around terrain traps.

Watch for the lack of an overnight refreeze of surface snow, wet snow deeper than boot top and initial pinwheels and initial small loose wet snow avalanches that indicate an increasing loose wet avalanche danger. It is always a good plan to be away from avalanche terrain by the warmest midday and afternoon hours. Careful snowpack evaluation and cautious routefinding will be essential through Tuesday.

Cornices should be starting to melt back but may remain unstable during prolonged warmer weather. Avoid potential cornices which can break well back from the edge along ridges and avoid traveling in areas underneath cornices on ridges above. See our blog post about cornices here.

Don't linger and move one a time if you decide to travel under rock slabs holding snow where sudden glide avalanches can release.

NWAC forecasts and statements do not apply to where conditions are likely to be more dangerous above the crest level on the volcanoes.

Avalanche Summary:

Recent Weather

The first half of last week was unusually cool with significant accumulating snowfall in the Olympics and Cascades, with the highest snowfall totals on or near the volcanoes. Strong upper level ridging over the last few days has allowed a steady increase in freezing levels over the weekend peaking to around 12000-13000 feet on Monday. Overnight temperatures have been well above freezing at NWAC weather stations the last few nights.

Recent Avalanche Observations

Loose wet slides on steeper slopes have been the story over the last few days as expected. Larger slides were observed in areas that received more recent snowfall last week.

Forecast for Tuesday:

Weather Outlook

Strong upper level ridging Monday and Tuesday will continue the recent warmth with freezing levels holding steady in the 11,000-14,000 ft range along with mostly sunny days and mostly clear nights. However, a deep low pressure system in the NE Pacific will approach the B.C. coast Monday night and shift the ridge inland on Tuesday, allowing for some cooling in the Olympics and north Cascades during the day on Tuesday. Crest level westerly winds are forecast to increase during the day Tuesday for all areas. As the upper low passes inland and to our north Tuesday night, much cooler air will push into the region along with marine clouds along the west slopes of the Cascades and Olympics and bring about a few light showers.

Avalanche forecast and travel advice

Many steeper aspects have already produced loose wet avalanches over the last few days. On steeper slopes that have not released yet, the loose wet avalanche potential remains highest during the warmer, sunny daytime hours. Large or very large loose wet snow avalanches are likely on the volcanoes where there has been the most recent snow. Remember that even small, loose wet snow avalanches can be powerful and dangerous especially around terrain traps.

Watch for the lack of an overnight refreeze of surface snow, wet snow deeper than boot top and initial pinwheels and initial small loose wet snow avalanches that indicate an increasing loose wet avalanche danger. It is always a good plan to be away from avalanche terrain by the warmest midday and afternoon hours. Careful snowpack evaluation and cautious routefinding will be essential through Tuesday.

Cornices should be starting to melt back but may remain unstable during prolonged warmer weather. Avoid potential cornices which can break well back from the edge along ridges and avoid traveling in areas underneath cornices on ridges above. See our blog post about cornices here.

Don't linger and move one a time if you decide to travel under rock slabs holding snow where sudden glide avalanches can release.

NWAC forecasts and statements do not apply to where conditions are likely to be more dangerous above the crest level on the volcanoes.

USE AT YOUR OWN RISK

This Backcountry Avalanche Forecast is provided in conjunction with the US Forest Service, and is intended for personal and recreational purposes only. Safe backcountry travel requires preparation and planning, and this information may be used for planning purposes but does not provide all the information necessary for backcountry travel. Advanced avalanche education is strongly encouraged.

The user acknowledges that it is impossible to accurately predict natural events such as avalanches in every instance, and the accuracy or reliability of the data provided here is not guaranteed in any way. This forecast describes general avalanche conditions and local variations will always occur. This forecast expires 24 hours after the posted time unless noted otherwise.